Using MODFLOW/MODPATH combined with GIS analysis for groundwater modelling in the alluvial aquifer of the River Sieg, Germany

Size: px

Start display at page:

Download "Using MODFLOW/MODPATH combined with GIS analysis for groundwater modelling in the alluvial aquifer of the River Sieg, Germany"

Bernard Francis
5 years ago
Views:

1 Models for Assessing and Monitoring Groundwater Quality (Proceedings of a Boulder Symposium July 1995). IAHSPubl.no. 227, Using MODFLOW/MODPATH combined with GIS analysis for groundwater modelling in the alluvial aquifer of the River Sieg, Germany WOLFGANG-ALBERT FLUGEL & CHRISTIAN MICHL Geographisches Institut, Universitât Jena, Lôbdergraben 32, D Jena, Germany Abstract Geographic Information Systems (GIS) have been increasingly applied for geohydrologic research studies as they provide a variety of spatial analysis for applications in groundwater modelling. In this study the GIS "IDRISI" was linked to the modularly structured groundwater model MODFLOW to simulate the hydrological dynamic of the alluvial aquifer of the River Sieg, Germany. Using a 10-year groundwater data base imported into the GIS "IDRISI" the hydraulic conductivity and the transmissivity of the aquifer were analyzed by raster overlay techniques and imported into PROCESSING MODFLOW and MODFLOW. After establishing the hydraulics of the aquifer, MODFLOW was calibrated for steady-state and transient conditions. Additionally, the tracking model MODPATH was used to detect conceptual errors in the assumed boundary conditions, and in the simulated infiltration from the rivers into the aquifer. MODFLOW simulated the groundwater dynamics in the aquifer with a good fit to the observed data for steady-state and transient conditions. By using the GIS raster data base and the PROCESSING module of MODFLOW an effective and improved pre- and post-processing was obtained. Future research will focus on the linkages (i) of the river dynamics, (ii) of the unsaturated zone with the aquifer, and (iii) of the MODFLOW model with the GIS system "GRASS". INTRODUCTION Groundwater is one of the major water resources in Germany. About 90% of the total groundwater consumption is provided by alluvial aquifers, which receive recharge by infiltration of rainwater and river bank storage. Considering the increasing demand of this water resource in the future groundwater protection against pollution from various types of land uses became a paramount issue in water resources management. Geographic Information Systems such like the PC-based GIS "IDRISI" (Eastman, 1992) provide a flexible toolset for water resources management (Fliigel & Liillwitz, 1993; Kovar & Nachtnebel, 1993), as data pre- and post-processing can be automated to a high degree. Additionally, the raster data base of the GIS permits an efficient data transformation between the spatial distribution represented in the GIS and

118 Wolfgang-Albert Flùgel & Christian Mich! the areal discretization obtained by the groundwater model using finite elements or finite differences.

2 118 Wolfgang-Albert Flùgel & Christian Mich! the areal discretization obtained by the groundwater model using finite elements or finite differences. Commonly used finite difference models in groundwater hydrology are based on the mathematical techniques developed by Trescott et al. (1976) or those of Pricket & Lonnquist (1971). MODFLOW (McDonald & Harbaugh, 1988) is a three dimensional, modularly structured open system groundwater flow model and is a further development of the Trescott model (Trescott et al., 1976). In this case study, the raster GIS "EDRISI", the finite element model MODFLOW and the particle tracking code MODPATH (Pollock, 1989) were applied to simulate the groundwater dynamics of the alluvial aquifer of the lower River Sieg supplying water for half a million people in the area near Bonn, Germany. OBJECTIVES The purpose of the study was to apply a combined groundwater modelling approach to a 10-year data set of the Sieg aquifer by linking a raster GIS with the MODFLOW groundwater model and to evaluate the obtained results. The objectives of the modelling exercise were: (i) to calibrate MODFLOW using the particle tracking program MODPATH for steady-state and transient conditions to the Sieg aquifer; (ii) to evaluate the performance of the MODFLOW modules RIV and STR1 describing the interaction of the aquifer and the River Sieg to delineate areas of different seepage dynamics; and (iii) to link the groundwater model with the raster GIS "IDRISI" and a data base. STUDY AREA The alluvial aquifer of the River Sieg, a tributary of the River Rhine, is located on the northern bound of the Rheinische Schiefergebirge, Germany. It is part of the tectonic subsidence of the Niederrheinische Bucht which started to decline in the Eocene (early period of tertiary). The tertiary clay as impervious layer builds the lower non-flow boundary condition underlying the alluvial gravel sediments which were accumulated in the pleistocene. Three different pleistocene systems of drainage basins can be distinguished. They are composed of gravel formations originating from the River Sieg and the River Rhine. The pétrographie spectrum of all three terrace systems is a homogeneous composition of white quartzes, sandstones and grey-white clayey shales. The average depth of the Pleistocene deposits is increasing from the lower to the middle terrace from 13 m to about 20 m. All of them are build up of mostly coarse gravel, which is well rounded. Inclusions of sandy layers are found in the two older terraces. Depending on the heterogeneity of the gravel deposits, hydraulic conductivity varies from 2.0 x 10" 2 to 2.0 x 10" 3 m s" 1. The climate of the study area is oceanic with a mean annual temperature of 10 C. Annual precipitation ranges between 650 and 750 mm with an average of 680 mm. The annual distribution is homogeneous with a moderate maximum in the summer and fall. Due to the surrounding mountain ranges the climatic conditions are favorable for

3 Using MODFLOW/MODPATH combined with GIS analysis for groundwater modelling 119 agriculture. The predominant land use, however, is made of the impervious areas of the settlements which have a great impact on the groundwater recharge. DATA BASE and GIS LINKAGE The geohydrologic data base was imported into dbase IV and was used by the GIS "IDRISI" and the groundwater model MODFLOW. Pre- and post-processing of this data base was done by using the two packages PROCESSING MODFLOW and MODPATH. The method used for creating the data base and carrying out the GIS analysis can be described as follows: (i) the geohydrologic data of a 10-year period ( ) including observations of groundwater heads in 200 wells, discharge measurements of the River Sieg and the River Rhine and climatological data were organized as a project data base in dbase IV; (ii) a digital elevation model (DEM) having a 50mx50m grid size resolution was imported into the GIS together with digitized geology, soil and land use maps; (iii) using overlay analysis maps of the hydraulic conductivity, the transmissivity and groundwater recharge areas were derived; (iv) over a data transfer module the GIS data base was linked with MODFLOW and PROCESSING MODFLOW and maps of simulated groundwater levels were imported into IDRISI. The linkage between the different software tools used in this study is shown in Fig. 1. DATA J- ( Spatial Data) 1 ( Point Data V- Geographic Information System IDRISI J Datamanagement L. 1 d-base P other GIS-Systeme GRASS / SPANS PROCESSING MODFLOW other Modelling Système OPUS / RZWQM MMS -J MODFLOW I Visualisation Corel DRAW PROCESSING MODFLOW Fig. 1 Schematic presentation of the data flow and the linkage between the GIS, the MODFLOW model and the geohydrologic data base. MODEL APPLICATION The finite element groundwater model MODFLOW was calibrated for the alluvial

120 Wolfgang-Albert Flùgel & Christian Michi geological border between tertiary clay and quartery deposits /"^-^ rivera Rhino, Sieg and Pleisbach 1 I finite difference grid (grid size 50, 100.

4 120 Wolfgang-Albert Flùgel & Christian Michi geological border between tertiary clay and quartery deposits /"^-^ rivera Rhino, Sieg and Pleisbach 1 I finite difference grid (grid size 50, and 300 m) production walls (S141 mentioned in text) UENOEN villages (adding up to 40 % of the landuse of the total study area) o observation wells (mentioned in text) Fig. 2 Study area of the lower River Sieg, Germany showing the model grid, the three referenced groundwater wells and the boundary conditions. aquifer of the River Sieg by calculating hydraulic heads as well as drawdowns for steady-state and transient conditions. Additionally, the water budget of the aquifer and particle travel times were calculated using MODPATH. The aquifer of the study area and its discretization is shown in Fig. 2. Its boundary conditions are given by the River Rhine in the west, and the River Sieg in the north (Dirichlet conditions) as well as by the barrier between the Tertiary sediments and the gravel deposits in the southeast (Neumann conditions). MODEL RESULTS Steady-state simulation For the steady-state simulation, the GIS-maps of the hydraulic conductivity and the land use dependent groundwater recharge were imported from the GIS to the model grid. Hydraulic conductivity was derived from several pumping tests and using the maps of geology and topography of the underlying tertiary bedrock. Groundwater

Using MODFLOW/MODPATH combined with GIS analysis for groundwater modelling 121 recharge was calculated in MODFLOW on a monthly basis using évapotranspiration rates of the Penman Monteith model.

5 Using MODFLOW/MODPATH combined with GIS analysis for groundwater modelling 121 recharge was calculated in MODFLOW on a monthly basis using évapotranspiration rates of the Penman Monteith model. Surface runoff on the flat valley floor and variations in the groundwater storage of the aquifer can be neglected for steady-state conditions. To account for the interaction between the aquifer and the two rivers Rhine and Sieg the MODFLOW modules RIV and STR1 were used to simulate seepage from the aquifer into the rivers and vice versa. The input parameters needed for this modelling task were derived from field data of gauging stations in the rivers Sieg and Rhine and the adjacent aquifer. They were optimized during different model runs. Groundwater recharge and the water level of the two rivers needed as model input are shown in Fig. 3.The following results were obtained by the steady-state simulation: (i) groundwater recharge consists of 30% recharge from precipitation and 70% of seepage from the River Sieg; (ii) inflow along the southeastern boundary is negligible; (iii) seepage from the River Sieg varies from about 90 1 s" 1 near the production wells to about 5 1 s" 1 in the northern and western parts of the aquifer. The flow paths computed by MODPATH indicate travel periods between five months near the production wells and about ten years at the southeastern boundary of the aquifer. Water Level (Sieg) Water Level (Rhine) Groundwater Recharge I Fig. 3 Monthly groundwater recharge and water level of the River Rhine and the River Sieg for the years 1984 to Transient simulation The transient simulation was based on the insight obtained by the steady-state model. Twenty-seven groundwater observation wells and three production wells were selected for the calibration exercise. For the summer water year the spatial groundwater recharge was adjusted to obtain the best fit between measured and simulated heads. Calculated specific yields varied between 28% for the younger alluvial deposits to 16% at the older alluvial terraces. From the evaluation of measured and simulated head time curves areas of different infiltration were delineated. Simulated and measured hydraulic

6 122 Wolfgang-Albert Flùgel & Christian Michl Years measured simulated Fig. 4 Head time curves of three reference wells showing the measured and simulated hydraulic heads. heads are shown in Fig. 4. Curve I at the production well S141 is reflecting the variation of hydraulic head in the River Sieg. The groundwater recharge is strongly influenced by the flow dynamics in the River Sieg. At the observation well S161 (curve II) this influence has decreased considerably, and curve III from well S204 shows the dynamics of the aquifer recharged only by infiltrating rainfall, indicated by the gentle amplitudes of the head time curve. All three curves, however, show a very good fit between simulated and measured hydraulic heads described by correlation coefficients of r=0.91 for curve I, r=0.89 for curve II and r=0.80 for curve III. The dynamics between river bed infiltration and adjacent aquifer obtained from the steady-state simulation could be verified during several transient runs. CONCLUSIONS AND FUTURE RESEARCH NEEDS The groundwater flow in the aquifer adjacent to the lower Sieg could be successfully simulated for steady-state and transient conditions using the finite difference model MODFLOW. An improved data pre- and post-processing was obtained combining the raster based GIS 'TDRISI" with PROCESSING MODFLOW and MODFLOW. The GIS based data model permits fast presentation and improved interpretation of the models input and output. The MODFLOW modules RIV and STR1 were successfully applied to delineate areas of different river bank seepage, and the stream-aquifer interaction was described very well by the model providing the input parameters could be estimated. The particle tracking program MODPATH was used to calculate travel paths to estimate solute transport and point-sources for groundwater pollutants. Based on the presented results the following research needs can be defined: (i) to improve the

Using MODFLOW/MODPATH combined with GIS analysis for groundwater modelling 123 simulation of the stream-aquifer interaction, the MODFLOW modules RIV and STR1 have to be linked with MODBRANCH (Swain,

7 Using MODFLOW/MODPATH combined with GIS analysis for groundwater modelling 123 simulation of the stream-aquifer interaction, the MODFLOW modules RIV and STR1 have to be linked with MODBRANCH (Swain, 1994); (ii) to delineate areas of different groundwater recharge from the valley floor, agro-hydrologic models like OPUS (Ferreira & Smith, 1992) or the Root Zone Water Quality Model (DeCoursey et al., 1992) have to be linked with MODFLOW. Acknowledgment The authors are pleased to acknowledge the cooperation with Mr. Keith Turner, School of Mines, Denver, USA, the U.S. Geological Survey, Denver, USA and the Wahnbachtalsperrenverband, Germany for the supply of the model and the model data base. REFERENCES Chiang, W.-H. & Kinzelbach, W. (1993) Processing Mod/low. Hamburg, Heidelberg. DeCoursey, D., Ahuja, L., Hanson, J., Shaffer, M., Hebson, C. & Rojas, K. (1992) Root Zone Water Quality Model Version 1.0: Technical Documentation. U.S. Department of Agriculture, Agricultural Research Service, Great Plains Systems Research Unit. Eastman, J. R. (1992) IDRISI Version 4.0. User's Guide and Technical Reference. Worcester. Fliigel, W.-A. & Liillwitz, T. (1993) Using a distributed model with GIS for comparative hydrological modelling of micro- and mesoscale catchments in the USA and in Germany. In: Macroscale Modelling of the Hydrosphere (ed. by W. B. Wilkinson) (Proc. Yokohama Symp., July 1993), 59-66, IAHS Publ. no Ferreira, V. A. & Smith, R. E. (1992) Opus, an integrated simulation model for transport of nonpoint-source pollutants at the field scale: vol. I, Documentation. U.S. Department of Agriculture, Agricultural Research Service, ARS-98. Kovar, K. & Nachtnebel, H. P. (eds) (1993) Application of Geographic Information Systems in Hydrology and Water Resources Management (Proc. Vienna Conf, April 1993). IAHS Publ. no McDonald, M. G. & Harbaugh, A. W. (1988) A modular three-dimensional finite difference groundwater flow model. USGS Techniques of Water-resources Investigations Report, book 6, chapter Al. Pollock, D. W. (1989) Documentation of computer programs to complete and display pathlines using results from the USGS modular three-dimensional finite difference groundwater model. USGS Open File Report Prickett, T. A. & Lonnquist, C. G. (1971) Selected digital computer techniques for groundwater resource evaluation. Illinois State Water Survey 55. Prudic, D. E. (1989) Documentation of a computer program to simulate stream-aquifer-interactions using a modular, finite difference groundwater flow model. USGS Open File Report Swain, E. D. (1994) Implementation and use of direct-flow connections in a coupled groundwater and surface-water model. Ground Water 32, Trescott, P. C, Pinder, G. F. & Larson, S. P. (1976) Finite difference model for aquifer simulations in two dimensions with results of numerical experiments. USGS Techniques of Water Research Investigations no. 7.

Using GIS, MODFLOW and MODPATH for groundwater management of an alluvial aquifer of the River Sieg, Germany

Using GIS, MODFLOW and MODPATH for groundwater management of an alluvial aquifer of the River Sieg, Germany HydroGIS 96: Application of Geographic Information Systems in Hydrology and Water Resources Management (Proceedings of the Vienna Conference, April 1996). IAHS Publ. no. 235, 1996. 551 Using GIS, MODFLOW